Wireless Local Area Networks (WLANs) have recently gained in popularity and importance. These networks are a special case of standard computer Local Area Networks (LANs), where the wires or optical fibers interconnecting computers have been partially or completely replaced by radio frequency (RF) data links. WLANs may also be viewed as a special case of commonly encountered cellular telephone networks, where the relatively large distances in the order of tens of miles covered by cellular telephones have been significantly reduced to hundreds of feet, (within buildings) in exchange for much higher data transmission rates. WLANs offer the possibility of interconnecting information technology devices at relatively high speeds without wires, and hence yield significant reductions in installation cost together with significant improvements in user convenience.
The increased usage and reliance upon WLANs has in turn dictated an increase in the level of performance and functionality testing that must be carried out in order to ensure that the WLAN protocol has been properly implemented and that WLAN equipment will function predictably, reliably and robustly under all circumstances. Without a significant level of testing, it is not possible to guarantee this reliability and performance.
In general, WLAN testing and characterization seeks to achieve the same goals as traditional wired LAN testing. The following categories of tests must be carried out:
(a) Performance measurements. These tests seek to determine some metrics of network performance, such as data rate (typically measured in bits per second), bit or frame error ratios, system utilization, latency, burst tolerance, etc. Performance measurements are normally carried out by transmitting test stimulus data at various speeds to the device under test and making quantitative measurements on the response from the device.
(b) Conformance tests. These tests attempt to quantify the level of adherence of the device to a set of specifications or requirements. These specifications may include published standards (e.g., the IEEE 802.11 standard for WLANs). They are usually carried out by presenting a set of well-defined patterns or stimuli to the device under test and verifying that the device responds as per the specification to each set of stimuli.
(c) Interoperability tests. These tests are used to ensure that two or more devices from different sources can intercommunicate without problems. Interoperability tests can be done by utilizing test equipment to simulate or emulate devices with which interoperability is desired, and measuring the responses of the device under test to ensure that the expected values are matched.
(d) Diagnostic tests. Diagnostic tests are performed during design and development to expose any faults or design errors in the device that may be causing performance, conformance or interoperability tests to fail. These are typically performed with the same test equipment, but set up instead to generate arbitrary stimuli to the device under test and record its responses.
Unlike wired LANs, however, WLANs have created a number of significant problems related to performance and functionality testing and characterization. The complexity of the protocol required to implement the high-speed data transfers is quite high, requiring correspondingly more complex and detailed testing capability. A single piece of WLAN equipment may need to communicate with multiple computers or end-stations, all sharing the same spectrum and in close proximity to each other. This renders the interactions observed between the equipment and the end-stations more complex and less predictable. The attenuation and multipath characteristics of the RF channel used to communicate between the WLAN equipment and the end-stations can cause significant variations in behavior and performance. Interference can also occur between WLAN equipment located in the same or adjacent buildings, even though they are physically isolated. All of these render the WLANs testing many times more complex than the testing for traditional wired or fiber-optic LANs.
Implementation and testing of WLAN networks, equipment and components is hence very different from that of standard wired LANs such as Ethernet. An important difference is that location is a significant parameter within the test environment; changing the location of equipment has virtually no impact on a wired LAN, but creates a huge impact on a WLAN system. Therefore, wired LAN testing methods and test equipment are quite unsuitable for WLAN testing, and a substantially new approach is required. This new approach, and the apparatus required to realize it, are the subjects of the present invention.
The key factor that distinguishes protocol testing of wireless data communication networks such as WLANs from that of wired LANs is the concept of location aware protocol testing. Location awareness here refers to the fact that the exact three-dimensional spatial location of a WLAN device, relative to the other WLAN devices and to the environment in which it is placed, is of great significance in the performance and functioning of the WLAN and must be accounted for. For example, shifting the position of a WLAN device by a few feet (e.g., placing it in a different room) can materially affect the throughput or interference seen by the device. This is clearly not the case for wired LAN technologies, which operate using copper or fiber optic links that are substantially insensitive to changes in physical location. This means that WLAN test systems must support the ability to account for the three-dimensional spatial location of the device or system being tested. In addition, the tests performed upon a device or system at a particular location cannot be reproduced at a different location without also duplicating some or all of the characteristics of the original location. Such duplication may be performed either directly (i.e., re-creating the physical environment of the device or system) or indirectly (i.e., by simulating the characteristics of the physical environment, without duplicating it in its entirety). The latter is clearly much simpler.